(4) Drug Interactions:
Aminosalicylic acid at a dosage of 12 grams in a rapidly available form has been reported to produce a 20 percent reduction in the acetylation of isoniazid, especially in patients who are rapid acetylators; INH serum levels, half lives and excretions in fast acetylators still remain half of the levels seen in slow acetylators with or without p-aminosalicylic acid. The effect is dose related and, while it has not been studied with the current delayed release preparation, the lower serum levels with this preparation will result in a reduced effect on the acetylation of INH.
Aminosalicylic acid has previously been reported to block the absorption of rifampin. A subsequent report has shown that this blockade was due to an excipient not included in PASER granules. Oral administration of a solution containing both aminosalicylic acid and rifampin showed full absorption of each product.
As a result of competition, Vitamin B12 absorption has been reduced 55% by 5 grams of aminosalicylic acid with clinically significant erythrocyte abnormalities developing after depletion; patients on therapy of more than one month should be considered for maintenance B12.
A malabsorption syndrome can develop in patients on aminosalicylic acid but is usually not complete. The complete syndrome includes steatorrhea, an abnormal small bowel pattern on x-ray, villus atrophy, depressed cholesterol, reduced D-xylose and iron absorption. Triglyceride absorption always is normal.
In one literature report 8 hours after the last dosage of aminosalicylic acid at 2 gm qid serum digoxin levels were reduced 40% in two of ten patients but not changed in the remaining eight.